1. Field of the Invention
The present invention relates to a display device, and in particular, to a display device where thin film transistors are formed on a substrate having a display portion.
2. Description of the Related Art
Liquid crystal display devices (panels) have a pair of substrates, which are positioned so as to face each other and sandwich liquid crystal in between as a housing, and have an image display portion formed of a large number of pixels that are aligned in a matrix with the liquid crystal as one component.
In addition, the pixels in the image display portion are usually driven in accordance with a so-called active matrix method. That is to say, pixel groups formed of a number of pixels aligned in the direction of rows are selected in sequence, and in accordance with the timing for this selection, a video signal is supplied through the drain signal line connected to all of the pixels aligned in the direction of columns. In this case, each selection of a pixel group is made by turning on the thin film transistors formed in the pixels that form the pixel group through a scan signal supplied through the gate signal line connected to all of the thin film transistors.
It is widely known that these liquid crystal display devices are provided with a scan signal driving circuit for supplying a scan signal to the gate signal lines, and a video signal driving circuit for supplying a video signal to the drain signal lines formed around the periphery of the above described image display portion on the substrate where the video display portion is formed, as well as a great number of thin film transistors in the respective driving circuits formed at the same time as the above described thin film transistors within the pixels.
These thin film transistors have a so-called MIS (metal insulator semiconductor) structure, and a so-called bottom gate type, where the thin film transistor is formed on the surface of the substrate on the liquid crystal side that faces the backlight from among the substrates in the panel, is known. In the bottom gate type thin film transistor, the gate electrode is formed so as to be located on the substrate side rather than on the semiconductor layer side, so that light from the backlight that is emitted through the above described substrate can be blocked by the above described gate electrodes, because the characteristics change when the above described semiconductor layer is irradiated with light.
In recent years, thin film transistors having a two-layer structure, where a microcrystal silicon layer and an amorphous silicon layer are layered in sequence on the gate electrode side in the semiconductor layer, have come into use (see JP2005-167051A). The microcrystal silicon layer can be formed by carrying out heat treatment on an amorphous silicon layer. This thin film transistor has higher field effect mobility than thin film transistors where the semiconductor layer is formed only of an amorphous silicon layer. In addition, the so-called S value (swing factor), which is one of the initial properties, can be made small, and effects can be gained such that chronological fluctuation in the threshold value voltage can be kept small.